For many printer systems of the kind described above, it is desirable to have two, three or more times as many print picture elements (pixels) per inch as there are print head printing elements (e.g. ink jet orifices) per inch. (Although the subsequent discussion will refer to the printing means as ink jets, they could be thermal printer elements, impact printer elements, or light emitter printer elements.) To achieve the higher resolution, the print head is indexed by appropriate amounts parallel to the linear direction of the array of jets. Each jet then can print at more than one pixel location on the line parallel to the array.
For example it might be desired to have a print resolution of 600 dots/inch (dpi) using a 300 jet/inch linear array print head. One known way to accomplish such a doubling of resolution is to do a simple two pass printing. On the first pass, dots are printed at 300 dpi parallel to the jet array and 600 dpi perpendicular to the array. The print head is indexed 1/600 inch and a second pass is printed. The print head is then indexed one array length and the same two pass printing sequence is again carried out. A disadvantage of this approach is that small jet to jet differences across the array can be accentuated because adjacent dot pairs are printed by the same jet, causing a banding artifact to be observable.
U.S. Pat. No. 4,622,650 discloses one approach to eliminate this banding artifact. Rather than use the two pass system described above where each pixel is addressed by a predetermined jet of the array, the '560 patent approach proposes a four or more pass scheme wherein each pixel location is addressable by two or more jets. A random or pseudo-random choice is made as to which of the two or more jets actually prints the pixel. This radomizing process helps to break up the visible banding.
However, the '560 patent technique has disadvantages. First, it slows down the printing process. Whereas the simple scheme for doubling the resolution required two printing passes, the '560 patent scheme requires four or more, halfing throughput. Second, besides the individual jet to jet differences of ink jet printers, there can be regional variations across the array which can affect several adjacent jets. These region variations can include, for example, air drag and fluid flow variations near the ends of the array and hole size variations due to orifice fabrication phenomena. The '560 patent approach of printing each pixel based on a random choice between adjacent jets, does not eliminate print banding caused by such region variations.
U.S. Pat. Nos. 4,009,332 and 4,198,642 describe different interlace approaches that reduce apparent banding by assuring adjacent pixels are not printed by the same or adjacent jets. However, the interlacing approaches described in these two patents each suffer a serious drawback. They do not allow for the simple doubling of pixel density when using an even number of addressable print elements. For printers ranging from impact printers to high resolution printers, a simple doubling of pixel density is often preferred over tripling or quadrupling. For most data system architectures, it is highly desirable to use 2.sup.n addressable print elements.